1. Field of the Invention
The present invention relates to a built-in input filter forward converter, and more particularly to a converter that exhibits non-pulsating input current.
2. Description of the Prior Art
Pulse-width modulation, quasi-resonant, multi-resonant and pulse-width modulation zero-voltage-transition forward converters have been proposed in the prior art to provide output voltage despite changes in load or input voltage. See [1] K. H. Liu and F. C. Lee, "Secondary-Side Resonance for High-Frequency Power Conversion," IEEE Applied Power and Electronics Conference Proc., 1986, pages 83-89. [2] W. A. Tabisz and F. C. Lee, "A Novel Zero-Voltage-Switched Multi-Resonant Forward Converter," High Frequency Power Conversion Conference Proc., 1988, pages 309-318. [3] H. J. Kim, C. S. Leu, R. Farrington, and F. C. Lee, "Clamp Mode Zero-Voltage-Switched Multi-Resonant Converters," IEEE Power Electronics Conference Record, 1992, pages 78-84. [4] G. Hua, C. S. Leu, and F. C. Lee, "Novel Zero-Voltage-Transition PWM Converters," IEEE Power Electronics Conference Record, 1992, pages 55-61.
A controller varies the duty cycle at a power MOSFET which is turned on and off in order to maintain constant the DC load voltage. Hence, a pulsating input current flows from power source. A typical example of this is a conventional prior art tertiary-winding forward converter shown in FIG. 8, where a transformer comprises two identical primary windings and a secondary winding with 1:1:N turns ratio, and "N" is a positive integer. FIG. 9 shows the key waveforms in the operation of the tertiary-winding forward converter of FIG. 8. As seen from FIG. 9, pulsating current results in getting a higher RMS value to cause additional losses and generating undesired current harmonics in the source. An input filter stage is thus required to alleviate those problems.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages of the conventional forward converter.